That's entirely possible, although fairly inconvenient. The requirement would be for your computer to be in the same room with your stereo.

If your computer has digital outputs, you're in business. Connect your computer to an outboard DAC and connect the DAC to your amplifier. Or, if you have a receiver with a DAC in it, then you could just use that.

I assume you're on a PC, given that you're using RealPlayer as your primary (?) music player. Check the back panel and look for either an orange color-coded RCA connector (coaxial digital) or a squarish socket, probably covered with a plastic cap or rubber flap. That is optical digital (toslink).

Either of the above could be tapped and routed into a high-quality DAC.

Another option is to buy an external/upgraded internal sound card with analog outputs. Many quality cards also have digital outputs, so the DAC option would still be viable.

The final (and poorest-sounding) option is to connect your computer directly to your amplifier with a Y-cable minijack-to-stereo RCA adapter.

Oh! I totally missed that! So, we're working with internet radio, then? Well, that's a completely different story. Streams typically have bitrates of 128 kbps or less, and will sound hollow and flat when played back on a nice system, or even decent computer speakers.

Try using the optical output again, but this time, rip a CD in a lossless or uncompressed format. It ought to compare to a CD.